1. Field of the Invention
The present invention relates in general to a lighting apparatus constituted by light emitting elements such as light emitting diodes. In particular, the present invention relates to a lighting apparatus which has such a simple construction as to be easily detached from and attached to a camera or the like, and which is capable of providing a stable luminous intensity independently of a change in temperature of light emitting elements.
2. Related Background Art
For example, there is known an inspection apparatus for photographing an object of inspection using a camera to carry out the quality control of the object based on the photography results. In such an inspection apparatus, in order to make an image of the object clearer, a lighting apparatus for lighting up an object is also simultaneously used. For this lighting apparatus, there is used one having a plurality of light emitting diodes (hereinafter referred to as “LEDs” for short when applicable) used therein because the LED has essentially the high directivity and hence does not require a reflecting plate or the like, and is miniature and has a low power consumption, and so forth.
However, the LED has the characteristics that a temperature thereof changes in correspondence to the use situation, and the quality of light thereof also changes in correspondence to the change in temperature. More specifically, the LED has a problem that when the temperature thereof rises, the quantity of light thereof decreases, and, for example, after the quantity of light becomes temporarily stable after the LED has been turned ON, the temperature rises with a lapse of the lighting time to decrease the quantity of light. For this reason, in the usual way, those LEDs are used while their temperatures are controlled by a temperature measurement device and a cooling device such as a Peltier element which are disposed in the vicinities of the LEDs. Exemplifications of such a construction are disclosed in Japanese Patent Application Laid-Open No. 2002-101274, Japanese Patent Application Laid-Open No. 2001-043728 and Japanese Patent Application Laid-Open No. 2000-337967, for example.
Examples of the above-mentioned inspection apparatus include an apparatus for visual inspection of semiconductor products (refer to Japanese Patent Application Laid-Open No. 10-206237, for example) and an apparatus for recognizing a shape of an article (refer to Japanese Patent Application Laid-Open No. 10-312450, for example). In such an apparatus, light having high directivity is applied to an object to photograph the object which is made clear by reflection of the light, and the inspection results, the shape recognition results, or the like are obtained based on the photography results. Possible objects of photography include one in which chips or the like are mounted on a circuit board. In the photography of such an object, in order to cope with the miniaturization of chips, the promotion of high density of a mounting state, and the like, in recent years, there is required a combination of a light source which can have higher directivity and provide the stable quantity of light, and a camera having high sensitivity or a camera with which color photography can be carried out.
In an inspection apparatus used in an inspection process for high density mounting electronic components obtained through the multikind and small-quantity production process, it is necessary to photograph clear images of various kinds of boards and mounting chips which are different in height, size, reflectivity, color tone, and the like, combinations thereof, and the like. Even when such a lighting apparatus changes in lighting state for an object, the quantity of change in lighting state has not become a problem in the normal photographing conditions up to this day, or the photography or the like has been carried out under a condition in which the quantity of change does not become a problem. However, for example, in the case where an electronic component is inspected for mounting state, when the electronic component, the mounting board, and the like are further miniaturized, and also the color tones or the like in their materials become close to each other, the above-mentioned change in lighting state may become unable to be disregarded.
For this reason, it is required that focal points of a lighting apparatus and a camera be readily and speedily changed, and lighting rays be applied to an object of the photography with the nearly equal quantities of rays and from nearly equal directions to obtain an image. It is conceivable that such a request will become more and more severe in the future along with the miniaturization and promotion of high performance of the electronic components. In addition, it is also supposed that when the camera and the lighting apparatus are miniaturized and the camera and the lighting apparatus are then tried to be easily detached from and attached to each other, the disposition itself of a temperature measurement device within the lighting apparatus becomes difficult.
In addition, in order to photograph and analyze an object of the photography which is more difficult in resolution, there is also used a camera for capturing an image in the color style. In the usual way, in such a camera, colors of an image are extracted based on a difference in radiation temperature between an object and the black body. For this reason, when a temperature of a camera itself rises due to the heat conduction from the LEDs or the radiation heat from the LEDs, there is a possibility that although there is a single object of the photography, the color of the resultant image differs from part to part of the image. Even if this color change does not become a problem in the current use situation, when an object of the photography is miniaturized and complicated, it is required to construct a photography system which takes that color change into consideration.
In the usual way, a focal point of a camera can be readily adjusted by driving a built-in lens of the camera. However, when a lighting apparatus constituted by a plurality of LEDs each having high directivity is used as a light source, it is normally difficult to move a focal point of the lighting apparatus. In addition, when it is required to change a luminescent color of an LED, normally, a filter or the like is forced to be disposed in front of the LED to cope with that request. Examples of a normally conceivable coping method include a method involving: preparing in advance a plurality of kinds of lighting apparatuses having different focal lengths; and fixing any one of these lighting apparatuses to a camera in correspondence to an application. Examples of the construction of the lighting apparatus include ones disclosed in Japanese Patent Application Laid-Open No. 2001-043728 and Japanese Patent Application Laid-Open No. 2000-337967.
As described above, however, in order to obtain the stable quantity of light from an LED, the LED needs to be used under the environmental conditions having a fixed temperature. For this reason, as described in Japanese Patent Application Laid-Open No. 2001-043728 etc., the lighting apparatus needs to be provided with a thermometer, a Peltier element as a cooling device, and a control system for the thermometer and the Peltier element. Hence, it is judged that in actual, this lighting apparatus will require a complicated wiring and the like. Consequently, when any one of the lighting apparatuses disclosed in the above-mentioned Patent Documents is tried to be installed in an actual camera, it is judged to be difficult to carry out an operation for the installation readily and speedily.
In addition, Japanese Patent Application Laid-Open No. 2002-101274 mentioned above discloses a construction in which an LED, a Peltier element, and a heat radiating plate are provided integrally with one another. Japanese Patent Application Laid-Open No. 2002-101274 also discloses the contents in which for the maintenance or the like, those constituent elements can be separated from one another. However, the construction disclosed in Japanese Patent Application Laid-Open No. 2002-101274 permits the constituent elements to be separated from one another only for the maintenance. Thus, for example, it is judged that this construction is not suitable for a situation in which an LED is frequently exchanged for another one. In addition, with the constructions disclosed in the above-mentioned Patent Documents, it is judged to be difficult to reduce a change in temperature which is generated in a camera or the like having the LEDs fixed thereto due to the fixing of the LEDs or the like.